perf tools: Add perf.data to .gitignore

Signed-off-by: John Kacur <jkacur@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: acme@redhat.com
LKML-Reference: <1259073301-11506-2-git-send-email-jkacur@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>